A coated type magnetic recording medium which is prepared by dispersing ferromagnetic particles such as ferromagnetic iron oxide particles, ferromagnetic chrome dioxide particles or ferromagnetic alloy particles in an organic binder such as a copolymer of vinyl chloride and vinyl acetate, a copolymer of vinyl chloride and vinylidene chloride, a cellulose type resin, an acetal type resin, a urethane resin or an acrylonitrile and butadiene type resin to prepare a coating composition, coating it on a non-magnetic support and drying it has been widely used. A thin metal film type magnetic recording medium having a ferromagnetic thin metal film as a magnetic recording lyaer, which is formed by a vapor deposition method such as vacuum evaporation method, a sputtering method or an ion plating method, has drawn attention with the recent increased demand for high density recordings. It is necessary in both coated type magnetic recording media and thin metal film type magnetic recording media that good contact between a magnetic layer and a magnetic head is provided, so that electromagnetic conversion properties are improved; therefore, in general, a smooth magnetic layer is desirable.
If a magnetic layer is excessively smooth, running properties thereof in a video tape recorder deteroriates. Consequently, running properties of backing layers need to be improved.
Therefore, a method for providing a coated film (backing layer) comprised of particles of inorganic pigment and thermoplastics resin on a back surface of a support is disclosed in Japanese Patent Publication No. 34324/79. However, in this method there is a disadvantage that a backing layer comprised of only thermoplastic resins is easily scratched and thus-scratched powders increase dropouts.
It is disclosed in Japanese Patent Application (OPI) No. 144604/76 (the term "OPI" as used herein refers to a "published unexamined Japanese patent application") that a thermoplastic resin and a trifunctional isocyanate compound are used as a binder for a backing layer in order to improve scratch resistance. However, this method is also economically disadvantageous because a long period of time for heat hardening the binder is necessary. There is also another disadvantage that when a hardening treatment is conducted, the magnetic recording medium is then wound onto a roll, whereby the rough surface of the backing layer is transferred to the magneti layer, thereby increasing the surface roughness of the magnetic layer, and deteriorating electromagnetic properties.
To overcome these disadvantages, it is disclosed in Japanese Patent Application (OPI) Nos. 40755/82 and 3132/83 and U.S. Pat. No. 4,419,406 that a backing layer can be hardened by radiation exposure.
However, the above techniques are not entirely satisfactory, because there is no consideration therein with respect to the support itself. That is, surface smoothness on a surface of a magnetic layer is deteriorated, whereby electromagnetic conversion properties of the magnetic layer are deteriorated. Additionally, as scratch resistance of a backing layer is insufficient, so that the backing layer is easily scratched, and therefore dropouts increase after repeated running of a tape.